Method and device for enhancing the signal-to-noise ratio of ECG signals

Abstract: In a method for enhancing the signal-to-noise ratio of ECG signals, A/D-converted signals from a plurality of cardiac cycles are time-synchronized to form a signal ensemble, whereupon the concordance between the signals in the ensemble is determined in time intervals, comprising at least two samples, during at least a part of the cardiac cycles. A device for enhancing the signal-to-noise ratio of ECG-signals, comprises means (4, 5) for recording, A/D-converting and other processing of the ECG-signals. A synchronization unit is arranged to time-synchronize the signals from a plurality of cardiac cycles to form a signal ensemble, and a calculator unit (12) controlled by a window unit is arranged to determine the concordance between the signals in the ensemble in time intervals, comprising at least two samples, during at least a part of the cardiac cycles.

Description: The present invention relates to a method and a device for enhancing the signal-to-noise ratio of ECG signals according to the preambles of claims 1 and 8 respectively. ECG signals recorded on the exterior of the body always contain extensive noise originating from muscular activity, digitalization of the recorded analog ECG-signal, the recording instruments and influence from the power supply. Using modern technology, however, the two latter sources of noise can be largely eliminated. But noise from the muscular activity of the patient and the digitalization procedure often drowns out low-amplitude signal components in the ECG signal. In an effort to reduce noise and, thus, improve the signal-to-noise ratio for these low-amplitude signal components, the ECG signals have hitherto been subjected to different kinds of averaging procedures. Time-varying Wiener filtration has also been attempted in order to enhance the signal-to-noise ratio, see Weerd and Martens: "Theory and Practice of A Posteriori Wiener Filtering of Average Evoked Potentials", Biol. Cybern., 30 , pp. 81-94, (1978), and Weerd and Kap.: "A Posteriori TimeVarying Filtering of Averaged Evoked Potentials", Biol. Cybern., 41 , pp. 223-234, (1981). This technic is based on the estimated signal and noise effects determined for different frequencies. In this process, some spectral leakage is unavoidable in the Fourier transform procedure utilized. With low signal-to-noise ratios, this method also results in consistently erroneous estimation of the signal in the noise. The object of the present invention is to propose a new method and provide a new device which makes possible additional enhancement of the signal-to-noise ratio which, in its turn, makes possible estimation of low-amplitude signal components in the ECG signal, such as so-called "late potentials", without use of Fourier transform with all its disadvantages. This object is achieved with a method according to claim 1 and a device according to claim 8. In the present invention representative parts of ECG-signals from a plurality of cardiac cycles are thus filtered in a filter the characteristics of which are related to the prevailing signal-to-noise ratio. Thus according to the invention, the signals from a plurality of cardiac cycles are time-synchronized to form a signal ensemble, and the correlation across the signal ensemble is determined, although not the time correlation. In this way the size of the resulting cross-covariance matrix is reduced such that determination of the optimum filter at every point of time is made possible. According to an advantageous embodiment of the device according to the invention, the correlation between the signals in the signal ensemble is determined in successive, overlapping intervals with the aid of a window unit. The optimum filter at any given moment is thus determined through short-period analysis. In this way time distortion is avoided (smearing). An exemplified embodiment of the device according to the invention will now be described in greater detail, referring to the accompanying drawings on which FIG. 1 is an overview block diagram of the device according to the invention; FIG. 2 is a block diagram showing a part of the device in FIG. 1 in greater detail and FIG. 3 shows curves illustrating the technic utilized by the invention. Recorded ECG signals are converted into digital form in the A/D converter 2, and QRS complexes are detected in the detector 4 in FIG. 1. Averaged signal values are formed in the averager 6, and an interval or time window, utilized in the subsequent unit 10 for enhancing the signal-to-noise ratio of ECG signal during at least a part of each cardiac cycle, is determined in the window unit 8. The units 6, 8 and 10 are described in greater detail with reference to FIG. 2. Recorded ECG-signals from a plurality of cardiac cycles are time-synchronized so they form an ensemble of signals. The elimination of low-frequency components, which is performed by a high-pass filter with a low cut-off frequency, is essential in the determination of the optimum filter at any given moment. This high-pass filtration, like any other signal processing, takes place in the signal processing unit 5. In the averager 6 average values over the signal ensemble are formed. Averaging needs not be performed as a "pure" averaging of all the signals in the ensemble, but can be carried out in different other ways in order to "improve" the average value and provide a representative signal cycle. So, e.g. strongly deviating signals in the ensemble could be omitted in the averaging, and the averaging could be performed e.g. in successive, overlapping intervals determined by the window unit 8. Averaging could also consist in median determination. In the correlation determination unit 12 the correlation between the individual signals in the ensemble of signals is determined according to a Maximum Likelihood method. The ensemble correlation thereby obtained is given by the following equation: in which z i denotes the i-th filtered ECG signal, N the number of cardiac cycles considered and M the number of samples in the interval or time window. The ensemble correlation thus determined is utilized for determining the optimum filter 14 for filtering the mean signal value obtained from the averager 6 in order to get as large a signal-to-noise ratio as possible. The optimum filter is determined for each successive interval or time window, whereby the resulting filter becomes time-dependent. The filter consequently has a certain weight for each observation interval or sample. The filtration in the unit 14 is thereby reduced to multiplication by a weight function h n obtained from the correlation determination unit 12. This is illustrated in FIG. 3 which shows N signals in the signal ensemble and the resulting weight function h n which determines the characteristics of the filter 14. The momentarily optimum filter realized in this way is a nonfrequency-selective filter, and the magnitude of the signalto-noise ratio is used for determining the filter weights. Since the signal-to-noise ratio is a function of the number of available cardiac cycles, also the momentarily optimum filter, and accordingly its output signal, becomes a function of this parameter. This filter can obviously vary considerably if the number of cardiac cycles is low. However, this variation decreases as the number of utilized cardiac cycles increases. With an increasing number of cardiac cycles N, the noise in the averaged signal decreases, a circumstances which contributes to enhancement of the signal-to-noise ratio at the same time as the momentarily optimum filter thus is improved through the access to more information, as mentioned above. Both these effects consequently contribute to a reduction of fluctuations in the output signal from the filter as the number of cardiac cycles increases.

Claims: Method for enhancing the signal-to-noise ratio of ECG signals, characterized in that A/D-converted signals from a plurality of cardiac cycles are time-synchronized to form a signal ensemble, whereupon the concordance between the signals in the ensemble is determined in time intervals, comprising at least two samples, during at least a part of the cardiac cycles, which concordance is employed to control subsequent filtering of the signals. Method according to claim 1, characterized in that the concordance is expressed in weights. Method according to claim 1 or 2, characterized in that the mean value is formed of the signals in said signal ensemble. Method according to any of the claims 1 - 3, characterized in that the determination of concordance comprises determination of the correlation between the signals in the signal ensemble. Method according to claim 4, characterized in that the correlation between the signals in said ensemble is determined in successive, overlapping intervals. Method according to claim 3 and 4, characterized in that the signal mean value is filtered according to a weight function determined by the correlation. Method of claim 6, characterized in that the weight function determines a filter with time variable characteristics for filtering the signal mean value. Device for enhancing the signal-to-noise ratio of ECG-signals, comprising means (2, 4, 5) for recording, A/D-converting and other processing of the ECG- signals, characterized in that a synchronization unit is arranged to time-synchronize the signals from a plurality of cardiac cycles to form a signal ensemble, and in that a calculator unit (10, 12) controlled by a window unit (8) is arranged to determine the concordance between the signals in the ensemble in time intervals, comprising at least two samples, during at least a part of the cardiac cycles, which calculator unit (10, 12) is adapted to provide an output dependent on the determined concordance to control subsequent filtering of the signals in a filter unit (14). Device according to claim 8 characterised in that the filter unit (14) operates to filter signals according to a weight function determined by the concordance. Device according to claim 8 or 9, characterized in that an averager (6) controlled by the synchronization unit or the window unit is arranged to form the mean value of the signals in said ensemble. Device according to any of the claims 8 - 10, characterized in that the calculator unit (12) is arranged, for the determination of the concordance, to determine the correlation between the signals in the signal ensemble. Device according to claim 11, characterized in that said window unit (8) is arranged to control the calculator unit (12) to determine the ensemble correlation in successive, overlapping intervals of the signals in said ensemble. Device according to any of the claims 10 - 12, characterized in that the calculator unit (12) generates a weight function which determines a time-varying characteristic for the filter unit for filtering the signal mean value. Device according to any of the claims 11 - 13, characterized in that the calculator unit (12) comprises a Maximum Likelihood estimator.

Inventors: ATARIUS ROOZBEH, OHLSSON THOMAS, SOERNMO LEIF, ATARIUS, ROOZBEH, OHLSSON, THOMAS, SOERNMO, LEIF, Sörnmo, Leif
Applicants: SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1994
Main classification: G06F17
IPC codes: G06F17, A61B5